1. Field of the Invention
This invention relates to a thermal printing head which is used for example to print on thermosensitive paper or to cause ink transfer from a thermal transfer ribbon or film onto printing paper. More particularly, the present invention relates to improvements in a thermal printing head of the type wherein a connector board is overlapped on a head circuit board for connection to external circuits.
2. Description of the Prior Art
As is well known, thermal printing heads are widely used in facsimile machines to print transmitted information on thermosensitive paper. The thermal printing head is also used in printers of the type wherein the ink of a transfer ink ribbon or film is thermally caused to be transferred onto printing paper.
There are various type of thermal printing heads which include line-type heads and matrix-type heads. The line-type thermal printing head has a row (line) of multiple heating dots, as disclosed for example in Japanese Patent Application Laid-open No. 63-151466 or No. 63-221055. The matrix-type thermal printing head has a multiplicity of heating dots arranged in a matrix, as disclosed for example in U.S. Pat. No. 3,855,448 to Hanagata et al.
The present invention is directed primarily to the line-type thermal printing head. To clarify the objects of the present invention, reference is now made to FIGS. 10 and 11 which show a typical line-type thermal printing head.
As shown in FIGS. 10 and 11, the prior art thermal printing head comprises an elongate head circuit board 100 adhesively mounted on an elongate support member 101 which is made of aluminum for example to serve also as a heat sink. The head circuit board 100 carries a longitudinal row (line) of multiple heating dots 102 positioned adjacent one longitudinal side of the head circuit board. The head circuit board also carries a comb-like terminal portion 103 located adjacent the other longitudinal side of the circuit board, and an array of drive IC's 104 for selectively driving the heating dots 102. Though not shown, the head circuit board is further formed with a sophisticated conductor pattern.
The printing head further includes a strip-like flexible connector board 105 which is reinforced by a backing 106 made for example of glass-fiber-reinforced resin. In an assembled state, the backing 106 rests directly on the support member 101, as shown in FIG. 11. The flexible board 105 has a front marginal portion projecting beyond the backing 106 to partially overlap the head circuit board 100. The underside of the projecting marginal portion of the flexible board 105 is formed with a comb-like terminal portion 107 in corresponding relation to the terminal portion 103 of the head circuit board 100.
An elongate presser cover 108 is arranged above the connector board 105 and fixed to the support member 101 by means of mounting screws 109 penetrating through the presser member and the connector board into engagement with the support member. The underside of the presser cover is provided with an elastic rod 110 for pressing the comb-like terminal portion 107 of the connector board into intimate contact with the comb-like terminal portion 103 of the head circuit board 100 when the mounting screws 109 are tightened.
In operation of the printer, the heating dot line 102 of the head circuit board 100 is held in intimate contact with thermosensitive paper backed up by a platen 114 (FIG. 11). To enable loading (or re-loading) of thermosensitive paper and/or maintenance of the printing head, one of the printing head and the platen is mounted on a pivotable part of the printer with the other mounted to a fixed part of the printer, so that the printing head and the platen are movable toward and away from each other. Thus, it is necessary to provide a guide means for guiding the platen into a predetermined position relative to the heating dot line 102 when the printing head and the platen are moved toward each other, thereby insuring good printing quality.
The guide means shown in FIGS. 10 and 11 includes a pair of L-shaped platen guides 111 positioned at the respective ends of the support member 101. Each platen guide 111 is formed with a guiding cutout 112 for removably receiving a corresponding shaft end of the platen 114, and fixed to the support member 101 by means of screws 113.
The prior art thermal printing head described above has a serious problem of bending during printing operation. The reason for such bending is as follows.
Generally, the support member (heat sink) 101 and the presser cover 108 are equally made of aluminum because this material is light and yet easily formed into any desired shape. Therefore, these two parts have the same coefficient of linear expansion. However, the support member 101 receives heat immediately from the head circuit board 100, whereas the presser cover 108 receives heat indirectly through the mounting screws 109 with a time lag. Thus, at the time of initiating the actuation of the heating dot line 102 or abruptly changing the actuating voltage, the heat transmitting time lag leads to a difference in the degree of longitudinal expansion between the support member and the presser cover at least before reaching the steady state.
According to the prior art arrangement shown in FIGS. 10 and 11, since the comb-like terminal portion 103 of the head circuit board 100 extends substantially over the entire length of the head circuit board, the flexible connector board 105 and the presser cover 108 must also have a length substantially equal to that of the head circuit board. Further, the entire length of the presser cover 108 must be fixedly mounted to the support member 101 by the mounting screws 109. Thus, when the presser cover and the support member are longitudinally expanded to different degrees, the thermal printing head as a whole bends longitudinally as a result of the so-called "bimetal phenomenon". Also troublesome is the fact that all of the mounting screws 109 must be tightened up in a well controlled manner to insure uniform electrical contact between the two terminal portions 103, 107 over their entire length, consequently prolonging the time required for manufacturing the thermal printing head.
The prior art thermal printing head has another problem with respect to the platen guides 111. Specifically, each platen guide 111 need be made of a hard metal such as an iron-containing alloy because the platen guide must be mechanically strong enough to withstand repetitive engagement with the platen shaft. On the other hand, the support member 101 should be preferably made of a light metal such as aluminum to realize weight reduction. Thus, the platen guide must be initially separate from the support member and later fixed to the support member by the screws 113. As a result, the total number of required components increases, and a longer time is necessary for assembly. Particularly, the platen guide must be strictly adjusted in position for insuring good printing quality, so that careful mounting of the platen guide is required with resultant increase in assembling time.
Theoretically, the support member 101 may be made of an iron-containing alloy and rendered integral with the platen guide 111. However, the support member 101 is required to be rigid (namely thick enough) to minimize the previously described thermal bending of the printing head. Thus if the support member is made of such an alloy, the overall weight of the printing head increases unacceptably. In reality, therefore, the support member should be made of a light metal such as aluminum without integrating with the platen guide.
U.S. Pat. No. 4,963,886 discloses an improved thermal printing head which incorporates a head circuit board carrying a comb-like terminal portion arranged only in a limited central length which is substantially smaller than that of the head circuit board itself. Due to such an arrangement, a flexible connector board is correspondingly reduced in length, and a presser cover is made to press the connector board into contact with the comb-like terminal portion of the head circuit board only in the limited central length of the head circuit board. Thus, though a plurality of mounting screws are used to mount the presser cover onto a support member (heat sink), only those screws located within the limited central length need be tightened up in a well controlled manner. The remaining screws may be inserted loosely in diametrically larger bores of the presser cover to allow longitudinal expansion of the presser cover independently of the support member.
According to the arrangement disclosed in the above-described U.S. patent, thermal bending of the printing head can be prevented since the presser cover is allowed to expand independently of the support member except for a portion located in the limited central length of the head circuit board. Further, those of the mounting screws located outside this limited central length need not be tightened in a well controlled manner, so that the printing head can be assembled in a shorter time than the prior art printing head of FIGS. 10 and 11.
However, the printing head of the above U.S. patent still remains to be improved in the following respects.
First, the support member is rendered relatively thick to be rigid. Thus, if the support member is longitudinally board supported on the support member cannot be brought into uniform line contact with the platen, consequently resulting in deterioration of the printing quality.
It is of course possible to realize uniform line contact between the head circuit board and the platen by increasing the spring load applied to the support member because the platen rubber can elastically deform for intimate contact with the head circuit board under the increased spring load. In this case, however, a large tension must be applied to the printing paper for feeding passage between the head circuit board and the platen, thereby requiring a high-torque motor for paper feed in addition to increasing the risk of paper tearing.
Secondly, the presser cover still has a length substantially equal to that of the head circuit board in spite of the fact that the length of the flexible connector board has been greatly reduced. Indeed, the presser cover has dual functions of pressing the flexible connector board onto the head circuit board and of covering an array of drive IC's which are sensitive to external shocks. Generally, the drive IC array extends substantially over the entire length of the head circuit board, and it is for this reason that the presser cover must remain long. However, it is very beneficial for weight reduction if the length of the presser cover is reduced at least partially. It is also advantageous for simplification of assembly if the total number of required mounting screws are reduced as a result of reduction in length of the presser cover.
In the third place, the support member (heat sink) is made of a relatively thick aluminum plate. Thus, separate platen guides need be screwed to the respective ends of the support member at the time of incorporating into the printer, so that the same problem as described in connection with the prior art printing head of FIGS. 10 and 11 is inevitable.